The mechanisms of gastroesophageal reflux are well known in the supine posture. No data exist regarding the sitting up position. In the vertical position, the forces which operate to drive reflux are different. In this study, the reflux events after meal in 20 patients with reflux esophagitis will be studied manometrically, in vertical and horizontal position. The results will help to clarify the mechanisms of reflux in the vertical position.